The storage and transfer of LNG poses significant challenges, particularly with respect to LNG vapor in the form of boiloff gas (BOG). While LNG is stored in a tank, the rate of BOG production is generally relatively low. However, the rate of BOG production is significantly increased upon LNG transfer into the tank, mostly due to heat from the sendout pumps and thermal losses in transfer. The variation in the amount of BOG in the tanks must be taken into account in the recondensation processes that recapture the BOG for delivery to high pressure (HP) pumps used to route sendout LNG to a vaporizer for pipeline delivery.
Typically, BOG recondensation processes employ recondensers having a relatively large volume to so allow combination of the BOG condensate with the LNG sendout to thereby form a subcooled liquid, but also to provide a surge volume for the combined LNG sendout to ensure a minimum flow rate to the high pressure pump. If the surge volume is inadequate, vapor may be introduced into the high pressure pump, which may cause cavitation in the pump, leading to component damage, decreased efficiency, and an ultimately shortened pump life. Vapor may also be introduced into the pump if the BOG condensate is a bubble liquid. Therefore, the large volume recondensers must receive an appropriate amount of LNG sendout to ensure that the combined LNG sendout contains no vapor. The variable production of BOG in the tank means variability in the amount of LNG sendout required in the recondenser operation and difficulty in optimizing the system for safe operation of the high pressure pump.
EP Publication No. 2372221A1 discloses a BOG recondenser having a bottom section that acts as a holdup drum for the high pressure pump. LNG sendout is introduced into the top section for recondensation and into the lower section, wherein the lower section receives up to half-maximum sendout, maintained by a level controller. A padding gas is then used to maintain pressure. While such recondenser may allow for at least some reduction in equipment size, the volume of held up liquid in the bottom section is still considerable.
U.S. Pat. No. 8,117,852 discloses methods and configurations for a system to unload LNG from a carrier into a storage tank. As the carrier is unloaded, the boiloff gas (BOG) in the storage tank is recondensed and sent to the storage tank. At the same time, BOG from the tank is processed in a condenser and partly recirculated to cool the incoming stream of LNG. The vapor in the resulting mixed stream is then separated and used to maintain the tank pressure on the carrier during unloading.
U.S. Pat. No. 7,493,778 discloses a condensing assembly that includes a BOG line to a traditional condenser, in which the BOG is condensed using direct contact of LNG from the sendout line. Control of the flow rate of the LNG used to condense the BOG is based entirely on active control of the liquid level in the condenser. The condenser is “ventless”; the pressure in the condenser is maintained by back pressure in the section of the sendout adjacent to the condensate line.
U.S. Application No. 2011/0056238 discloses an LNG storage and regasification plant that reliquifies BOG from the tank, uses one portion of the BOG as fuel gas, recycles another portion of the BOG back to the storage tank for tank pressure and Wobbe index control, and feeds a further portion of the BOG to the sendout line. This system uses a conventional recondenser, such as described above, on yet another BOG portion prior to sending a stream of LNG to high pressure pumps for subsequent vaporization.
These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
These systems described above fall short of a comprehensive and effective solution to all of the problems associated with BOG during LNG storage and transfer, especially as it relates to combination of the BOG condensate with the LNG sendout, and accommodation of large volume changes in the BOG. Therefore, there is still a need for improved configurations and methods of BOG handling.